In the aluminum alloy extrusion business, there are a variety of extruded articles of various cross-sections. There has been an attempt to standardize language used to describe various extruded article configurations and such standardized language will be used here in describing the types of articles the invention is useful in extruding. Generally an extruded shape does not include a rod, bar or tube. A semi-hollow extruded shape includes a void which has a gap along its extruded length. A hollow extruded shape has a closed void along its extruded length. An extruded tube is of almost any cross-sectional shape such as round, square, rectangular, hexagonal, octagonal or elliptical where the tube is generally of uniform wall thickness.
Continuing efforts are being made to improve the dimensions of extruded semi-hollow and hollow shapes and tube in order to broaden their use in a variety of applications. The greater the dimensional accuracy of the extrusion, the greater the opportunity there is to replace milled pieces with extruded pieces of various aluminum alloys. There is also a continuing effort to enhance the speed of aluminum alloy extrusion while minimizing scrap and reducing die temperature build up. Such advances are described by Paul Robbins in "Advanced Design and Manufacture of Hollow Dies or Improved Extrusion Speeds" Proceedings of the 6.sup.th International Aluminum Extrusion Technology Seminar Volume 2, May 14-16 1996- Chicago Ill. Various die designs are discussed which enhance the speed of extrusion while limiting the temperature of extrusion. One of the difficulties encountered in increasing extrusion speed is mandrel deflection which is discussed by Luigi Ingraldi in "Design and Correction of Hollow Dies in Europe" Proceedings of the 5.sup.th International Aluminum Extrusion Technology Seminar Volume 1, May 19-22, 1992 Chicago, Ill. Improved mandrel designs reduce mandrel deflection and hence provide a better quality product.
When drawing the extruded product, problems are described in respect of U.S. Pat. No. 4,148,207 and Gunar Bilen in "Die Design and Die Correction as Affected by Pullers" Proceedings of the 4.sup.th International Extrusion Technology Seminar, Volume 2, Apr. 11-14, 1988, Chicago, Ill. For purposes of cold drawing, the mandrel or the die may be provided with a tapered section to improve the quality of the drawn product. As described in the article by Bilen pullers cause a significant problem with extended mandrel designs. Excessive bearing surface on the mandrel can cause the section to stick to the mandrel surface when puller force is applied, which can disastrously affect the shape of the extrusion.
PCT published application WO 97/02910 describes an extrusion die for hollow extrusions where the die cavity includes a preform chamber which allows the die to operate at increased velocities.
In the field of plastic extrusion various post-forming operations of the extruded plastic tube are described in U.S. Pat. Nos. 2,814,071; 3,963,403 and 4,983,347. Although the devices of these particular U.S. Patents function reasonably well for various purposes in post-forming of plastic articles, such post-treatment may be readily conducted on plastics because of their flexibility and inherent elasticity.
From a practical stand point, most aluminum extruders attempt to improve the accuracy of the die so as to reduce scrap, increase velocity of extrusion and control die temperature. However, post-forming operations are still necessary and which are carried out at a different station. Such post-forming operations include drawing, sizing, rolling, annealing, straightening and the like to achieve the desired accuracy with respect to semi-hollow and hollow extruded shapes and tube. Such additional post-forming techniques are expensive, time consuming and involve considerably more labor.
In accordance with an object of an aspect of this invention, an aluminum extrusion process and an aluminum extrusion die are provided which achieve the objectives of increasing extrusion speed, controlling extrusion die temperature and providing a hollow extrusion with internal dimensions having enhanced tolerances.